The Arctic Oscillation (AO), shown below, is an important Arctic climate
index with positive and negative phases, which represents the state
of atmospheric circulation over the Arctic. The positive phase (red) brings
lower-than-normal pressure over the polar region, steering ocean storms
northward, bringing wetter weather to Scotland and Scandinavia, and drier
conditions to areas such as Spain and the Middle East. While the value
of the AO index was strongly positive in the early 1990's compared to
the previous forty years, the value of the AO has been low and variable
for the last nine years. The year to year persistence of positive or negative
values and the rapid transition from one to the other is often referred
to as "regime-like".